What is third degree heart block?

What is Third-Degree Heart Block?

Third-degree heart block (complete heart block) is a complete absence of electrical conduction from the atria to the ventricles, where no atrial impulses reach the ventricular conduction system, resulting in independent atrial and ventricular rhythms. 1

Pathophysiology and ECG Characteristics

Complete dissociation between atrial and ventricular activity defines this condition, with the ventricles relying entirely on an escape pacemaker for rhythm generation. 2

The location of the escape rhythm determines both the ECG appearance and clinical severity:

• Junctional (AV nodal) escape rhythms produce narrow QRS complexes with ventricular rates of 40-60 bpm and are generally more stable 2
• Ventricular escape rhythms produce wide QRS complexes with rates of 20-40 bpm and carry higher risk of hemodynamic instability 2

Clinical Significance and Risk Stratification

The anatomic location of the block critically determines prognosis and urgency of intervention. 1

Intranodal (Proximal) Block

• Occurs at the AV node level 1
• Associated with stable junctional escape rhythms 1
• Not immediately life-threatening 1
• Monitoring should be considered on an individual basis 1

Infranodal (Distal) Block

• Occurs in the His-Purkinje system 1
• Can progress rapidly and unpredictably 1
• Associated with sudden death 1
• Requires continuous arrhythmia monitoring until pacemaker implantation 1

Common Etiologies

The most common cause is ischemic heart disease, with up to 20% of MI patients developing some conduction disturbance and 8% developing complete heart block. 2

Additional causes include:

• Myocarditis and infectious endocarditis 2
• Infiltrative cardiac diseases (sarcoidosis, amyloidosis) 3
• Non-ischemic cardiomyopathy 2
• Electrolyte disturbances 2
• Medication effects (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics) 3, 2
• Congenital heart disease 3, 4
• Neuromuscular diseases 3, 4
• Blunt chest trauma 5

Clinical Presentation

Patients may be asymptomatic or experience serious symptoms related to bradycardia, ventricular arrhythmias, or both. 1

If no escape rhythm generates, patients develop asystole and cardiac arrest. 2

Symptoms can include:

• Syncope 1, 2
• Hypotension and hemodynamic compromise 2
• Fatigue and exercise intolerance 3
• Signs of low cardiac output 6

Pediatric Considerations

Third-degree AV nodal block can occur in infants and children without cardiac surgery. 1

• Permanent pacing decisions are based on escape rate, heart rate, and symptoms associated with bradycardia 1
• For newborns, the ability to feed without hemodynamic compromise determines pacing need 1
• During assessment, neonatal ICU monitoring with continuous ECG is standard 1
• Thirty percent of congenital AV blocks remain undiscovered until adulthood and may present during pregnancy 3

Genetic Considerations

Familial clustering occurs with autosomal dominant inheritance patterns in idiopathic cases. 4

• Mutations in genes affecting transcription, excitability, and energy homeostasis have been identified 4
• Associated heart disease is common, including congenital malformations and cardiomyopathy 4
• Genetic models incorporate reduced penetrance and variable expressivity 4

Critical Pitfalls

Complete heart block must be distinguished from second-degree AV block - in third-degree block, there is complete absence of AV conduction, whereas second-degree block shows some conducted beats. 1

The incidence of high-grade AV block in adults over 65 is approximately 1 in 600 annually, making recognition and treatment essential. 2

Early diagnosis is critical as complete heart block may lead to sudden death, particularly when associated with infranodal disease. 1, 7